When sewing hems with invisible stitches on sewing machines of the whipstitch type, a workpiece guiding device is commonly used which includes a hemming element that is adapted to fold the marginal portion of the workpiece in the required manner, and a straight guide aligned with the sewing axis which is effective in guiding said workpiece parallel to said sewing axis. The hemming element is usually made so that it will move upwardly away from the hem being formed on tubular material just prior to completion of seaming when the initial portion of the seam again approaches the needle. This feature serves to assist the operator in preventing the sewn portion of the workpiece from again entering the sewing zone.
Additionally, to compensate for changes in sewing conditions and to facilitate adjustment of the device to accommodate such changes, known devices usually include a means for effecting movement of the straight guide in a direction which places it at a greater distance from the sewing axis. During the actual sewing operation the thickness of the workpiece material is frequently subjected to areas of increased thickness which can be attributed to such previous operations such as cross-sewing. For this reason during normal operation of the machine, the elements comprising the guiding device are maintained in a pre-selected position so that the needle of the machine is caused to pass through the thickness of the margin of the folded material in order that the thread associated with the needle is not visible within the fold when the sewn material is spread out. When these portions of the workpiece of increased thickness arrive at the guiding device, certain components thereof are caused to assume a slightly different position so as to accommodate these portions of said workpiece and avoid a binding condition as it passes between the tip of the hemming element and the hem guiding member. By avoiding such a binding condition a stretching of the material is prevented which is responsible for a slight shifting of the workpiece to the left of the sewing axis which is sufficient to prevent the needle from cooperating with that portion of the fold of the workpiece so as to satisfactorily perform its intended function. In the known forms of guiding devices the means for displacing those components for accommodating the portions of a workpiece of increased thickness is accomplished either manually by such means as a pressure plate or automatically by the actuation of electromagnetic or pneumatic control means. Actuation of the control means is accomplished automatically by a movable feeler element which engages the material and which is located in a narrow passage provided between the tip of the hemming element and the straight guide. At the moment when the increased thickness of material engages the tip of the hemming element, the movable feeler is urged in the direction of the straight guide, against the biasing force of a spring, and arrives at a position to effect actuation of the control means. The control means causes movement of said straight guide in a position where it is temporarily held for a sufficient length of time to permit those portions of the workpiece of increased thickness to reach and pass beyond the needle while maintaining alignment of and the required type of stitching in said workpiece. Although the spring operatively associated with the movable feeler element is of the light and sensitive type, it still causes a certain amount of resistance to those increased thickness portions of the workpiece as they are caused to advance through the guide and create a greater stretched condition to said workpiece than that which is considered an acceptable amount.
Although the use of the hemming element is essential to effect movement of the movable feeler element and hence the actuation of the control means, this element is not essential in forming the hem in the manner described above, for the operator of the machine manually controls the quantity of marginal material for forming said hem and is the actual creator thereof. Additionally the hemming element presents an undesirable requirement in the sewing cycle, for near the completion of said cycle, said hemming element must be manually displaced so as to prevent the initial portion of the sewn hem from making contact therewith. The procedure of displacing the hemming element necessitates what is considered excessive non-productive time for the operator must stop the sewing in progress and then with one hand displace said hemming element as required.
An object of the present invention is to eliminate the above disadvantages by providing a device in which the hemming element is replaced by an element adapted to press the first fold only of the marginal portion of the material on the needle plate and which is disposed so as not to interfere with the passage of the initial portion of the hem already sewn, i.e. such as not to require the direct intervention of the operator.
A further object is that of providing the device with a movable feeler element that is extremely sensitive to the presence of increased thicknesses of the workpieces so as to perform its intended function without causing damage to the material to be sewn.
The invention defines an invisible stitch hemming device especially adapted for hemming tubular materials which is characterized by the hemming element that defines a substantially horizontal plate having a tip portion that is insertable within the open upper fold of the hem and which is movable out of the upper fold under the thrust of the sewing as the initial sewn portion again approaches the needle.
Another desirable feature of the invention is that the feeler is disposed so as to project upwardly through the needle plate at a location beneath the tip of the hemming element.
These and other objects of the invention will become more fully apparent by reference to the appended claims and as the following detailed description proceeds in reference to the figures of drawing wherein: